Prostate cancer is very prevalent in old age, with approximately one half of all males over age 70 having been shown to develop prostatic cancer. This high incidence of prostate malignancy has led to the search for markers which may be used for its detection. The elevation of serum acid phosphatase activity in patients having metastasized prostate carcinoma was first reported by Gutman et al. in J. Clin. Invest. 17: 473 (1938). In cancer of the prostate, prostatic acid phosphatase is released from the cancer tissue into the blood stream with the result that the total serum acid phosphatase level greatly increases above normal values. Numerous studies of this enzyme and its relation to prostatic cancer have been made since that time, e.g., see the review by Yam in Amer. J. Med. 56: 604 (1974). However, the measurement of serum acid phosphatase by conventional spectrophotometric methods often fails to detect prostatic cancer in its early stages. In general, the activity of serum acid phosphatase is elevated in about 65-90 percent of patients having carcinoma of the prostate with bone metastatis; in about 30 percent of patients without roentgenological evidence of bone metastasis; and in about only 5-10 percent of patients lacking clinically demonstrable metastasis.
Prior art attempts to develop a specific test for prostatic acid phosphatase have met with only limited success because techniques which rely on enzyme activity on a so-called "specific" substrate cannot take into account other biochemical and immunochemical differences among the many acid phosphatases which are unrelated to enzyme activity of prostate origin. In the case of isoenzymes, i.e. genetically defined enzymes having the same characteristic enzyme activity and a similar molecular structure but differing in amino acid sequences and/or content and therefore immunochemically distinguishable, it would appear inherently impossible to distinguish different isoenzyme forms merely by the choice of a particular substrate. It is therefore not surprising that none of these prior art methods is highly specific for the direct determination of prostatic acid phosphatase activity; e.g. see Cancer 5: 236 (1952); J. Lab. Clin. Med. 82: 486 (1973); Clin. Chem. Acta. 44: 21 (1973); and J. Physiol. Chem. 356: 1775 (1975).
In addition to the aforementioned problems of non-specificity which appear to be inherent in many of the prior art reagents employed for the detection of prostate acid phosphatase, there have been reports of elevated serum acid phosphatase associated with other diseases, which further complicates the problem of obtaining an accurate clinical diagnosis of prostatic cancer. For example, Tuchman et al. in Am. J. Med. 27: 959 (1959) have noted that serum acid phosphatase levels appear to be elevated in patients with Gaucher's disease.
Due to the inherent difficulties in developing a "specific" substrate for prostrate acid phosphatase, several researchers have developed immunochemical methods for the detection of prostate acid phosphatase. However, the previously reported immunochemical methods have drawbacks of their own which have precluded their widespread acceptance. For example, Shulman et al., in Immunology 93: 474 (1964) described an immunodiffusion test for the detection of human prostate acid phosphatase. Using antisera prepared from a prostatic fluid antigen obtained by rectal massage from patients with prostatic disease, no cross-reactivity precipitin line was observed in the double diffusion technique against extracts of normal kidney, testicle, liver and lung. However, this method has the disadvantages of limited sensitivity, even with the large amounts of antigen employed, and of employing antisera which may cross-react with other, antigenically unrelated serum protein components present in prostatic fluid.
Chu et al. in International Patent Application Publication No. WO 79/00475, the contents of which are incorporated by reference herein, describe a new method for the detection of prostatic acid phosphatase isoenzyme patterns associated with prostatic cancer which obviates many of the above drawbacks. However, practical problems are posed by the need for a source of cancerous prostate tissue from which the diagnostically relevant prostatic acid phosphatase isoenzyme patterns associated with prostatic cancer are extracted for the preparation of antibodies thereto.
In recent years considerable effort has been spent to identify enzyme or antigen markers for various types of malignancies with the view towards developing specific diagnostic reagents. The ideal tumor marker would exhibit, among other characteristics, tissue or cell-type specificity, and would be released into the circulation or other biological milieu which is easily obtained from individuals. Previous investigators have demonstrated the occurrence of human prostate tissue-specific antigens.
For example, R. H. Flocks et al., I. J. Urol. 84: 134 (1960) immunized rabbits with an extract of BPH (benign prostatic hypertrophy) prostate tissue and showed the presence of tissue specific antiprostatic antibodies by a gel diffusion technique. However, they presented no data at all to indicate the nature of the reactive antigen. The precipitation lines formed by antiserum and prostate extract appear due to the reaction of prostatic acid phosphatase and its antibodies.
R. J. Ablin et al., J. Immunol. 104: 1329 (1970) and R. J. Ablin, Cancer 29: 1570 (1972) have also demonstrated the occurrence of human prostate tissue-specific antigens. By using antiserum obtained from immunizing rabbits with extracts of normal prostate, Ablin et al. showed two antigenic components in human prostate. One of these was identified as prostatic acid phosphatase, while the specificity of other was shown to be a non-prostatic tissue antigen. The xenoantibodies reactive to the second antigen could not be abolished by treating the antiserum with human prostatic fluid. Furthermore, this antigen was shown to be deficient in benign and malignant prostatic tissues. In contradistinction, the prostate antigen of the present invention is present in all prostate tissue, (normal, benign or malignant) in almost equal amounts. Further, it is detectable in prostatic fluid and cultured human prostatic malignant cells and its medium as well. The Ablin articles describe absorption of antisera to the antigens described therein with prostatic fluid, after which a precipitin line was still detected. Absorption of antibodies against the present antigen with prostatic fluid gives no precipitin line, indicating that the present antigen is present in prostatic fluid while that of Ablin et al. is not.
C. W. Moncure et al., Cancer Chemother. Rep. 59: 105, (1975) also demonstrated the occurrence of a human prostate tissue-specific antigen preparation which does not bind to DEAE sepharose at pH 8.0, as does the antigen of the present invention. This characteristic most probably indicates a significant structural difference between the present protein and that of Moncure et al.
Thus, there is still a need for simple, reliable, sensitive and specific reagents and techniques to detect prostatic cancer with acceptable diagnostic accuracy and without the aforementioned difficulties of the prior art. The present invention fills such needs.